The increasing occurrence of extreme weather conditions, such as the recent deluge in Mumbai, points to a dangerous threat - climate change. This is the first of a series of articles on human-induced climate change

It rained all day. It rained like it had never rained before. Trains stopped, cars were submerged, several died, and hundreds and thousands of people waded through the streets of Mumbai. The city that never stands still came to a grinding halt. It almost sounds like a scene from a sci-fi film, but in fact it is scarily real. Mumbai witnessed the strongest rains ever recorded in India in July 2005. Such catastrophic weather phenomena are often seen as acts of God, and they might well be, but the increasing occurrence of extreme weather in India and around the world points towards a dangerous threat - climate change.

Though floods, droughts, storms and other extreme weather events have always been a reality, they have been rare occurrences interrupting long periods of calm -- sudden outbursts marring nature's largely gentle rhythm. Now, because of human-induced climate change, that gentle rhythm is breaking up. Overwhelming scientific evidence indicates that climate change is real - the world is warming up and climate systems are changing. Findings from the Intergovernmental Panel on Climate Change (IPCC), which has been established by the World Meteorological Organisation (WMO) and the United Nations Environment Program (UNEP), show that the global average surface temperature increased by 0.6ÂºC over the course of the 20th century. Scientists have recorded the 1990s as the hottest decade in the world since the industrial revolution began. As a result of global warming, snow extent has decreased by about 10% since the 1960s, while mountain glaciers have retreated rapidly. The global average sea level rose by 10 to 20cm during the 20th century, and the amount of heat stored in the ocean has measurably increased since observations began in the 1950s.

Rainfall patterns have also changed in the Northern Hemisphere, with generally more rain at high latitudes and near the equator and less in the sub-tropics. Warm El NiÃ±o (which causes droughts and flooding) episodes have been more frequent, persistent and intense since the mid-1970s than during the previous 100 years.

One of the most important features of the IPCC Third Assessment Report is that it strengthens the conclusion that human activity is driving the observed climate change. The atmospheric concentration of CO2 is now 31% higher than it was in 1750, the highest it has been for the past 20 million years -- and it's accelerating. About three-quarters of the increase is from fossil fuel burning, while the rest is mostly due to deforestation. Atmospheric methane has increased even more dramatically, by 151% since 1750. Nitrous oxide and synthetic greenhouse gases (halocarbons) also continue to rise.

Much of this discussion - halocarbons and methane -- sounds like technical babble to most people. While it may seem like something esoteric that only scientists in white coats need to contend with, its impact on ecosystems, economies and local weather is real. Throughout the 10,000-year history of human civilisation, weather patterns have remained relatively constant, but the frequency of extreme weather events has increased steadily over the 20th century. The number of weather-related disasters during the 1990s was four times that of the 1950s, and cost 14 times as much in economic losses. These trends confirm the predictions of computer models: as the atmosphere warms, the climate will not only become hotter but much more unstable. Extreme events are likely to increase, and droughts and floods will become more common in many regions. Many alpine glaciers will disappear, snow cover and sea ice extent will continue to wither, and sea level is projected to rise. Climate change also raises other important concerns. How will our health be affected by global warming, how will agricultural practices change, how will wildlife cope?

Climate change is an issue that threatens the entire globe. However, it disproportionately affects developing countries like India and it will be most disruptive to the poorest of the poor - those who have the least resources and the least capacity to cope. With its huge and growing population, a long, densely populated and low-lying coastline, and an economy that is closely tied to its natural resource base, climate change could have potentially devastating impacts on India . The average temperature change is predicted to rise by 2 to 4Â° C with a doubling in CO2 concentrations. With climate change, rainfall patterns are also set to change. Western and central areas could have up to 15 more dry days each year, while in contrast, the north and north-east are predicted to have five to 10 more days of rain annually. In other words, dry areas will get drier and wet areas wetter. In an almost sadistic twist of events, climate change will make India more susceptible to both droughts and flooding. IPCC findings indicate that there will be an increase in the frequency of heavy rainfall events in South and Southeast Asia . Studies have also shown that the impact of snow melting in the high Himalayas will lead to flood disasters in Himalayan catchments.

The most dramatic effects of climate change will manifest in agriculture and forestry. These changes in turn could have profound implications for livelihoods and food security. Agriculture and allied activities continue to be fundamentally dependent on the weather in India . IPCC and other studies suggest that there will be a decrease in yields, though the percentage of decrease varies across different scenarios. Higher temperatures reduce the total duration of a crop cycle by inducing early flowering, and the shorter the crop cycle, the lower the yield per unit area. Climate change is also likely to have substantial impacts on forestry. Climate is an important determinant of the geographical distribution, composition and productivity of forests. Therefore, changes in climate could alter the configuration and productivity of forest ecosystems. In a case study of Kerala ( Achanta A and Kanetkar R,1996), results indicate that under the climate change scenarios, soil moisture is likely to decline and in turn reduce teak productivity from 5.40 m3/ha to 5.07 m3/ha. The study also shows that the productivity of moist deciduous forests could decline from 1.8 m3/ha to 1.5 m3/ha. Changes in forestry could potentially result in extinction of some species and loss of biodiversity.

The impact on water resources is also expected to be severe. India is considered rich in terms of annual rainfall but these resources are unevenly distributed, causing spatial and temporal shortages across regions. Climate change and variability are likely to worsen the problem of water scarcity that many parts of India face. Under a changed climatic regime, the combined effect of lower rainfall and more evaporation would have dire consequences. Both these would lead to less runoff, substantially changing the availability of freshwater in the watersheds. Also, potential changes in temperature and precipitation might have a dramatic impact on the soil moisture and aridity level of hydrological zones. With changes in the flows, annual runoff, and groundwater recharge, water available for usage will further decrease. Most major river basins across the country are likely to become considerably drier. One assessment (Hadley Centre Model Simulations) indicates that by the year 2050, the average annual runoff in the river Brahmaputra will decline by 14%.

Sea level rise associated with climate change threatens India 's low-lying and densely populated coastline which extends about 7,500 km. UNEP identifies India among the 27 countries that are most vulnerable to sea level rise. Most of the coastal regions are agriculturally fertile, with paddy fields that are highly vulnerable to inundation and salinisation. Coastal infrastructure, tourist activities, and onshore oil exploration are also at risk. The impact of any increase in the frequency and intensity of extreme events, such as storm surges, could be disproportionately large, not just in heavily developed coastal areas, but also in low-income rural areas. A case study of Orissa and West Bengal ( IPCC, 1992) estimates that in the absence of protection, a one-metre sea level rise would inundate 1,700 km of predominantly prime agricultural land. The economic implications of such a rise could be huge - ranging from Rs 2287 billion in the case of Mumbai, to Rs 3.6 billion in the case of Balasore. (TERI, 1996)

Climate change has other impacts which may seem less obvious at first, but would have very serious socio-economic consequences. For instance, some reports predict that India will be more prone to malaria, as changing weather patterns will result in potential breeding grounds for malarial mosquitoes at higher altitudes. Adverse weather patterns will also affect large-scale infrastructure projects that are designed to have a long life span. The recently constructed Konkan railway, a major infrastructure project laid through the high rainfall mountain region in mid-western India, is a typical example of a high-value long-life asset exposed to climate extremes.

The science of climate change is not a hundred percent accurate and different models and simulations suggest different scenarios. But there are certain facts that all scientists are unanimous about - the earth is getting warmer and climate systems are changing, and the impact of climate change is something that we are already contending with. What is also clear is that human activity has been responsible for this. It is unfortunate and, perhaps, unfair that globally the impact of climate change will disproportionately harm developing nations such as India despite the fact that we have contributed relatively little to cumulative greenhouse gas emissions. But we can't afford to sit around and cry foul. If the recent flooding in Mumbai and other parts of India are anything to go by, we need to get our act together fast. Because with climate change -- when it rains, it pours.

(This article is primarily based on findings from the IPCC, and specific studies have been cited. Aditi Sen works on Community Driven Development & Participation and Civic Engagement. She is with the Social Development Department of the World Bank, Washington.)